Add echo and line modes to Stdin.

runtime/bin/io_impl_sources.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # This file contains some C++ sources for the dart:io library.  The other
 # implementation files are in builtin_impl_sources.gypi.
 {
   'sources': [
     'crypto.cc',
     'crypto_android.cc',
@@ skipping 29 matching lines @@
     'secure_socket.cc',
     'secure_socket.h',
     'secure_socket_unsupported.cc',
     'socket.cc',
     'socket.h',
     'socket_android.cc',
     'socket_linux.cc',
     'socket_macos.cc',
     'socket_win.cc',
     'stdin.cc',
+    'stdin.h',
+    'stdin_linux.cc',
   ],
   'conditions': [
     ['dart_io_support==1', {
       'sources!' : [
         'filter_unsupported.cc',
         'secure_socket_unsupported.cc',
       ],
     },{  # else dart_io_support == 0
       'sources!' : [
         'filter.cc',
         'filter.h',
         'net/nss_memio.cc',
         'net/nss_memio.h',
         'secure_socket.cc',
         'secure_socket.h',
       ],
     }],
   ],
 }